RELATIONS OF DISINTEGRATION AND DECOMPOSITION. 501 



longer the journey the farther this process continues. However, as shown 

 on pages 432-434, there appears to be an inferior limit in size, beyond 

 which attrition ceases; that is, the particles become so fine that they are 

 floated and have not sufficient weight and momentum to materially grind 

 one another. This conclusion is further supported by the fact, given by 

 Judd, that the small particles of the alluvial deposits of the Nile which 

 have been transported great distances are angular, while the larger particles 

 of the same deposits are subangular or rounded. However, he attributes 

 the rounding of the larger grains to eolian rather than river erosion ; but this 

 does not lessen the force of the arg-ument as to there being a lower limit 

 beyond which the particles are not rounded by river action. Further 

 evidence in the same direction is the angular character of the loess of the 

 Mississippi Valley. Chamberlin and Salisbury found that the loess of that 

 valley, much of which must have been floated for long distances, probably 

 hundreds of miles, is wholly composed of angular particles which show no 

 evidence of wear. For the most part these loess grains are less than 0.005 

 mm. in diameter, although in some places 1 per cent or more exceed this 

 diameter, and a few grains are nearly 0.1 mm. in diameter. 



REGIONS FAVORABLE TO PROMINENCE OF DECOMPOSITION. 



The regions favorable to decomposition are those of humidity, low 

 latitude, moderate topographic relief, abundance of plants and animals, 

 and remoteness from the sea. 



Humid regions. — It is clear that humidity is one of the requisite conditions 

 for regions of rapid decomposition, since chemical action is Very slow in 

 the absence of water. In humid regions, as already shown, the belt of 

 weathering always contains a large and variable amount of water, and 

 water is essential for vigorous chemical action. Hydration is one of the 

 chief reactions of the belt of weathering, and in humid regions there is 

 abundance of the necessary compound for the process. 



High humidity may occur at any latitude and at any elevation. At 

 low latitudes and low elevations the temperature is high and life is abun- 

 dant, and these conditions are especially favorable to chemical work. 



As evidence of decomposition in the humid regions, Hilgard contrasts 

 the soils of such regions with those of the arid regions, and finds that in the 

 humid regions the soils are usually loams with a clay subsoil. The presence 



